A fuel cell device generates electricity directly from a fuel source, such as hydrogen gas, and an oxidant, such as oxygen or air. It does so by continuously changing the chemical energy of a fuel and oxidant to electrical energy. Since the process does not "burn" the fuel to produce heat, the thermodynamic limits on efficiency are much higher than normal power generation processes. In essence, the fuel cell consists of two catalytic electrodes separated by an ion-conducting membrane. The fuel gas (e.g. hydrogen) is ionized on one electrode, and the hydrogen ions diffuse across the membrane to recombine with the oxygen ions on the surface of the other electrode. If current is not allowed to run from one electrode to the other, a potential gradient is built up to stop the diffusion of the hydrogen ions. Allowing some current to flow from one electrode to the other through an external load produces power.
The membrane separating the electrodes must allow the diffusion of ions from one electrode to the other, but must keep the fuel and oxidant gases apart. It must also prevent the flow of electrons. Diffusion or leakage of the fuel or oxidant gases across the membrane leads to explosions and other undesirable consequences. If electrons can travel through the membrane, the device is fully or partially shorted out, and the useful power produced is eliminated or reduced. Ehrenberg et al. U.S. Pat. No. 5,468,574 discloses such a membrane which allows the diffusion of ions, but prevents both the flow of electrons and the diffusion of molecular gases. This membrane is also mechanically stable.
In constructing a fuel cell, it is particularly advantageous that the catalytic electrodes be in intimate contact with the membrane material. This reduces the "contact resistance" that arises when the ions move from the catalytic electrode to the membrane and vice versa. Intimate contact can be facilitated by incorporating the membrane material into the catalytic electrodes. [See Wilson and Gottsfeld J. Appl. Electrochem. 22, 1-7 (1992)]
For reasons of chemical stability, fuel cells presently available typically use a fully fluorinated polymer such as Dupont Nafion Registered.TM. as the ion-conducting membrane. This polymer is very expensive to produce, which raises the cost of fuel cells to a level that renders them commercially unattractive.
Ion-conducting polymers are well known. (See Vincent, C. A., Polymer Electrolyte Reviews I, 1987). The known polymers are, for the most part, similar to sulfonated polystyrene because of the known ability of sulfonated polystyrene to conduct ions. Unfortunately, uncrosslinked, highly sulfonated polystyrenes are unstable in the aqueous environment of a fuel cell, and do not hold their dimensional shape.
U.S. Pat. No. 4,849,311 discloses that a porous polymer matrix may be impregnated with an ion-conducting polymer to produce a fuel cell membrane. However, the ion-conducting polymer must be dissolved in a solvent which "wets" the porous polymer. When the solvent evaporates, there is sufficient porosity remaining in the porous polymer/ion-conducting polymer composite material that molecular oxygen can leak through to the fuel gas and result in an explosion.
U.S. Pat. No. 3,577,357 (Winkler) discloses a water purification membrane composed of block copolymers of sulfonated polyvinyl arene block and alpha-olefin elastomeric blocks. In one example a styrene-iosprene-styrene triblock copolymer was selectively hydrogenated, then sulfonated using a premixed SO3/triethylphosphate reagent at 60.degree. C. for 1.5 hrs. A sulfonated styrene-(ethylene-propylene) copolymer was the result. The method provided solid agglomerates of the polymer which were rolled on a mill to remove water, swelled in cyclohexane, slurried in an isopropyl alcohol/water mixture, and coagulated in hot water. No membrane was produced, and we have found that polymers produced according to the method of Winkler cannot be cast into films.
Gray et al. [Macromolecules 21, 392-397 (1988)] discloses a styrene-butadiene-styrene block copolymer where the ion-conducting entity is a pendant short-chain of poly(ethylene oxide) monomethyl ether (mPEG) complexed with LiCF3SO3 salt and connected through a succinate linkage to a flexible connecting entity which is the butadiene block of the triblock copolymer. The ion-conducting entity in the butadiene block is in the continuous phase of the polymer, and the areas populated by the ion-conducting entities do not preferentially touch each other to form continuous ion-conducting domains. This morphology does not facilitate the ion-conducting properties that are necessary for fuel cell operation. The styrene block functions only as a mechanical support structure for the polymer. Moreover, the molecular design chosen by Gray et al. is incompatible with the working environment of a fuel cell. Because the succinate linkage which joins the mPEG to the butadiene backbone and the ether linkages which join the ethylene oxide units are subject to cleavage by acid hydrolysis, these linkages are unstable in the low pH environment of a fuel cell even for short periods of time.
In the art of battery separators, as exemplified by U.S. Pat. No. 5,091,275, a number of porous polymers and filled polymer materials are well known. The pores of these polymers and composite materials are filled with, typically, a liquid electrolyte to conduct ions from one electrode to another in a battery. However, these battery separator materials allow the passage of gases, so that fuel cells made with them have an unfortunate tendency to explode as the oxygen leaks into the hydrogen side of a fuel cell.
To be useful, the hydrogen gas produced must be stored for later use to provide energy when needed. The production of hydrogen from water generally consists of transmitting electrical energy to electrodes within an electrolyzer to induce an electric potential difference which disassociates water into hydrogen and oxygen. The electrolyzer generally contains pure water having as electrolyte of sodium hydroxide or potassium hydroxide. These electrolytes are not destroyed nor do they need to be replenished during the operation of the electrolyzer. Thus, even though the electrolysis action (the producing of chemical changes by the passage of an electric current through an electrolyte (a nonmetallic electric conductor in which current is carried by the movement of ions, or a substance that when dissolved in a suitable solvent or when fused becomes an ionic conductor)) may take place intermittently, the hydrogen produced can be maintained in storage and turned back into electrical energy (either by combustion or by use of a fuel cell) when desired.
One of the more efficient electrolyzers presently available is a solid polymer electrolyte ("SPE") unit. These units basically consist of two electrodes, an anode and a cathode, placed in a perfluorinated sulfonic acid polymer. The electrodes are connected through an external circuit to a power supply. Water is broken down at the anode into oxygen, hydrogen ions and electrons. The electrons flow through the external circuit to the cathode while the hydrogen ions flow through the electrolytic polymer to the cathode where they combine with the electrons and form hydrogen. The equations at the anode and cathode are: EQU H.sub.2 O.fwdarw.2H.sup.+ 1/2O.sub.2 2e.sup.- EQU 2H.sup.+ 2e.sup.-.fwdarw.H.sub.2
and the overall reaction is: EQU H.sub.2 O.fwdarw.H.sub.2 1/2O.sub.2
The by-product of this process is an effluent containing trace hydrofluoric acid, oxygen gas and excess water.
SPE electrolyzers are one of the two main types of electrolyzers available. SPE electrolyzers are also known as PEM, or Proton Exchange Membrane, for the way in which they split water. The other type, liquid electrolyte ("LE") electrolyzers, uses as its electrolyte a strong acidic or basic solution, typically potassium hydroxide. However, there are a number of advantages that an SPE electrolyzer has over LE electrolyzers. The concentration of the solution in an LE electrolyzer must be maintained at a constant level for the electrolytic reaction to take place, while SPE electrolyzers maintain constant concentration over their life. SPE electrolyzers are also safer, since they do not require a supply of a strong highly corrosive basic solution as do LE electrolyzers.
The hydrogen gas thus produced is a storable, transportable, clean, and non-polluting fuel. However, hydrogen has the fundamental limitation of being difficult to store. Hydrogen has a boiling point of -252.87.degree. C. and a density of 0.09 grams per liter. This means that in order to store hydrogen in reasonable sized tanks, it must be stored either under pressure, at low temperature, or both. Unfortunately, it takes energy to create high pressures and low temperatures. Thus, the overall efficiency and cost effectiveness of producing and storing hydrogen is reduced.
In order to overcome the hydrogen storage problem, it has been found that hydrogen can be stored in a solid form via "rechargeable" metal hydrides, such as iron-titanium-manganese (Fe44Ti55Mn5) alloy, mischmetal-nickel aluminum hydriding (Mn0.97Ni4.5A10.5) alloy, and the like. This can best be described by the reversible chemical reaction of a solid metal hydride(Me) with gaseous hydrogen (H2) to form a solid metal hydride (MeH x): EQU 2/x Me H.sub.2.revreaction.MeH.sub.x heat
The forward or exothermic reaction is characteristic of the charging (absorption) of hydrogen to the hydride while the reverse or endothermic reaction is the discharging (desorption) of hydrogen from the metal hydride. Among the many advantages of hydrogen storage via a metal hydriding alloy, the most significant is the low charging and discharging pressures required to hydride which lessens the risk of leakage and explosion associated with storing hydrogen as a compressed gas.
When examining the thermodynamic aspects of the reversible metal-hydrogen reaction, it is advantageous to determine the absorption and desorption properties of metals from pressure-composition isotherms. The abscissa of such isotherms is typically in the form of a hydrogen atoms to metal atoms ratio ("H/M"). FIG. 1 shows the ideal absorption-desorption pressure-composition isotherm for a metal-hydrogen system where the plateau pressure ("P.sub.p ") 30 is shown connecting points B and C. Once the plateau pressure is reached, the majority of the absorption or desorption of hydrogen takes place at this constant pressure P.sub.p. The curves connecting points A and B as well as points C and D show that for a large increase or decrease in pressure, the amount of hydrogen absorbed or desorbed is small.
In reality, while such isotherms as shown in FIG. 1 might be achievable, most hydrides deviate from this ideal behavior. In addition to the fact that the plateau region slopes and the boundaries of this region are not as well defined, there also exists hysteresis between absorption and desorption curves. For ideal hydrides, there is no means by which to measure the composition of the hydride when located along the plateau pressure; but the slope in the isotherm for real metals makes finding the hydrogen to metal ratio as simple as knowing the temperature and pressure of the hydride.
The plateau pressure P.sub.p is related to the absolute temperature of the reaction, T.sub.R, by the Van't Hoff equation: ##EQU1##
where .DELTA.H is the change in enthalpy, .DELTA.S is the change in entropy and R.sub.u is the universal gas constant. From the Van't Hoff relationship one can determine the charging and discharging pressures and temperatures of the tank.
In recent years, numerous cars have been designed in order to realize a reduction of pollution and noise. Some of these cars have been fueled with nitrogen oxide and hydrogen, resulting in exhaust containing no carbon monoxide. Also, some of these cars have been getting driving force by loaded storage cells and motors.
However, there are several shortcomings of the cars previously made. First, these cars, which were fueled by hydrogen, have proven to display reduced driving force due to combusting hydrogen with an internal combustion engine. Second, regarding the exchange of fuel in the tank which stores the hydrogen and the refilling of this tank, there are currently serious problems concerning the potential for dangerous explosions. Third, these cars did not obtain sufficient driving distances per tank of fuel. Fourth, a car using a hydrogen fuel cell and a motor could not get enough cell capacity with the prior fuel cells such that it was necessary to combine many cells in order to get sufficient power. Finally, the prior fuel cells required a very long time to charge and their running distances were short.
It has also been previously suggested that in order to most efficiently use a fuel cell system for vehicular propulsion, the system should be, preferably, sized so as to provide sufficient power, at a useful voltage, for normal continuing operation, or cruising operation, when utilizing air as the oxidant, and that during peak loads, pure oxygen should be substituted for air as the oxidant. This allows the fuel cell system to be sized for normal low power/air operation, but also to provide a peak power capacity, at a suitable voltage, significantly greater than for normal operation, and without any complex changes to the system. Such a system is disclosed in U.S. Pat. No. 4,657,829. In this prior patent, the water generated by operation of the fuel cell is electrolyzed during normal operation by the excess electrical capacity of the fuel cell. The electrolysis results in the generation of hydrogen and oxygen gases, which in turn are stored under pressure for use when required at peak power capacity. Although this system does result in the desired peak power availability, the amount of oxygen which must be stored in order to have adequate peak power capacity is a problem for a vehicle for which minimum design weight is desired.
It is thus an object of the present invention to provide a fuel cell power system for a vehicle with improved peak power capability but with minimized high pressure gas storage requirement. It is yet a further object of the present invention to provide a fuel cell power system utilizing power created during operation of the vehicle and water generated by operation of the fuel cell to generate oxygen and hydrogen for use during peak power intervals, but wherein the effectiveness of the oxidant air is enhanced by enrichment with oxygen so as to reduce the amount of storage capacity required for peak acceleration requirements. It is yet another and further objective of the present invention to provide a fuel cell powered vehicle having improved efficacy during operation. Other objects and advantages will become apparent when considering the following specific description of an example of the invention.
The present invention provides a novel and useful power device which has overcome the problems existing in the prior hydrogen fuel cell systems used for electric cars, including but not limited to reducing exhaust pollution.